Project Summary Our long term goal is to characterize, expand and improve knowledge of the developmental and neurobiological determinants of sexual partner preferences/orientation using domestic sheep as the experimental model. The sheep is an exceptional model because variations in male-typical sexual partner preference occur spontaneously, with as much as 8% of the ram population exhibiting mating preferences for the same-sex. Sheep have a long period of gestation making them an ideal experimental model for the study of possible links between fetal neuroendocrine programming and adult sexual behavior. Similar to humans, sheep possess a sexually dimorphic preoptic nucleus (SDN) that is larger in males that are attracted to females than in males that are attracted to other males. The ovine SDN develops prior to birth and is enlarged by testosterone (T) exposure during a prenatal critical period that occurs after the external genitals have sexually differentiated. We recently discovered that the hypothalamic-pituitary-gonadal (HPG) axis is active during the critical period suggesting that variations of T exposure in males could be centrally mediated through control of gonadotropin releasing hormone (GnRH) secretion. In adults, GnRH neuron activity is regulated by inputs from afferent neurons that co-express kisspeptin (KP), neurokinin B (NKB) and dynorphin (DYN) and are referred to as KNDY neurons. KNDY neurons are reciprocally interconnected and critical to the control of pulsatile GnRH secretion and steroid feedback. We reported that KP, NKB, DYN and their receptors are expressed in the fetal lamb hypothalamus. However, whether a functional circuit is established among KNDY neurons and GnRH neurons in the fetal lamb brain is not yet known. The goal of this proposal is to test the hypothesis that KNDY-GnRH signaling plays a pivotal role in the activation/regulation of the reproductive axis during fetal development and is responsible for masculinizing sexual partner preferences by controlling T secretion by the fetal testes. My laboratory and collaborators have extensive expertise with the ram model and with the methods needed to elucidate the mechanisms involved in programming the functions of the brain that control neuroendocrine functions and sexual attraction. Our specific aims include: 1) Behavioral studies of rams prenatally treated with a GnRH antagonist to determine if GnRH control mechanisms that regulate fetal T secretion play a direct role in establishing adult male-typical sexual preferences; 2) Physiological studies using chronically catheterized non-anesthetized lamb fetuses to determine if KP, NKB and DYN regulate GnRH neurons and consequently pituitary LH and testicular T secretion during the critical period; 3) Anatomical studies to determine when during early development the neuronal components of the KNDY-GnRH neural circuit are established and how they are regulated. Our sheep model is unique because it allows us to identify mechanisms underlying sexual partner preference without the confound of having subjects that differ in sex as well as in sexual orientation. Advancing the sheep model bridges research interests of NICHD, NIMH and NINDS.